PROJECT SUMMARY Young adults have the highest prevalence of past-year depression and current tobacco use in the United States. Co-occurrence of depression and tobacco use is particularly problematic during young adulthood as nicotine addiction is solidified in this developmental period, and elevated levels of depressive symptoms promote sustained tobacco use and undermine quitting. Yet, there is limited research on young adults and evidence for the strength and direction of associations between depressive symptoms and tobacco use is inconclusive. Inconclusive findings indicate there may be heterogeneity in the patterns of associations between depressive symptoms and tobacco use across subgroups of young adults. Yet, few studies examine this heterogeneity. Further, most studies focus on cigarettes, although other products, such as electronic nicotine delivery systems, are increasingly prevalent and the concurrent use of two or more products (i.e., poly tobacco use) is more prevalent than single product use among young adults. Use of newly introduced vape pods, such as JUUL, is increasingly concerning. But, the role of vape pod use in the depressive symptoms-tobacco use association is not known. There is a critical need for longitudinal studies that identify for whom and under what conditions depressive symptoms are associated with use of all types of tobacco, and with transitions in use, such as from current to no tobacco use and vice versa. The primary objectives of this R01 are to determine if: 1) the role of depressive symptoms in tobacco use trajectories and transitions vary for subgroups across young adulthood, ages 18-34 and 2) the strength and direction of associations vary. We will identify moderating factors that capture the heterogeneity, and define for whom and under what conditions depressive symptoms are longitudinally associated with tobacco use outcomes. We will examine sociodemographic, intrapersonal, and contextual moderating factors, and all types of tobacco products and outcomes, including initiation, sustained use, addiction, cessation, poly tobacco use, and transitions in use. To do so, we will use eight waves of existing data collected in 2014-2019 from 5,482 initially 18-29 year old college students. The Specific Aims are to: 1) Identify the role of depressive symptoms in longitudinal tobacco use trajectories and transitions, and elucidate the temporal associations between depressive symptoms and tobacco use; 2) Examine the moderating roles of sociodemographic, intrapersonal, and contextual factors in the longitudinal trajectories, transitions, and temporal associations; and 3) Determine the role of vape pods, like JUUL, in the longitudinal tobacco use trajectories, transitions, and temporal associations. The expected outcome is identification of subgroups most vulnerable to initiating and continuing tobacco use, given elevated depressive symptoms. Results can be used to tailor tobacco interventions and determine to whom they should be targeted.